The invention relates to a switch mode power supply for a communication device.
Typically, a switch mode power supply (SMPS), includes a switching power transistor having a controllable duty cycle that is controlled by a duty cycle modulated signal. An alternating current (AC) mains supply voltage source is coupled to a rectifier for producing an input supply voltage for energizing the SMPS. Typically, a large input filter capacitor is coupled at an input of the SMPS for filtering AC components from a rectified input supply voltage produced in the rectifier. It may be desirable to eliminate the large input filter capacitor.
A typical SMPS requires the generation of a periodic switching signal to establish the timings of the duty cycle modulated signal. It may be desirable to utilize the periodic waveform of the mains supply voltage to establish the timings of the duty cycle modulated signal. Thereby, SMPS operation can be obtained without an added circuit complexity associated with the generation of the periodic switching signal.
In a SMPS, embodying an inventive feature, a mains supply voltage source is coupled to a rectifier for producing an input supply voltage. The rectified input supply voltage is coupled unfiltered to an input of the SMPS. A switching power transistor having a controllable duty cycle is controlled by a duty cycle modulated signal for producing a regulated output supply voltage from the rectified input supply voltage. The periodic waveform of the mains supply voltage is used to establish the timings of the duty cycle modulated signal.
In carrying out an inventive feature, in each cycle, current flow is initiated in the transistor, when the transistor is already fully turned on and a voltage developed between its main current conducting terminals is low or close to zero volts. Thereby, power dissipation is, advantageously, small. When the output supply voltage attains a threshold level the transistor is turned off.
In carrying out another inventive feature, hysteresis is provided for preventing the transistor from turning on again in the same cycle, after it has been turned off. Thereby, advantageously, the transistor is prevented from turning on again in the same cycle, when the voltage developed between its main current conducting terminals is no longer close to zero volts. Consequently, increased power dissipation is, advantageously, prevented.